Use active project filter as default field value when creating new tasks

Summary: See T389. If you're looking at tasks in project X, default new tasks
into project X.
Test Plan:
  - Created a new task without associated projects.
  - Created a new task from a filter view with 1 and 2 projects; defaults were
filled in.
  - Submitted task with invalid title, projects were correctly preserved.

Reviewed By: skrul
Reviewers: skrul, hunterbridges, jungejason, tuomaspelkonen, aran
CC: aran, skrul, epriestley
Differential Revision: 757

src/applications/maniphest/controller/taskedit/ManiphestTaskEditController.php

@@ -47,6 +47,11 @@ class ManiphestTaskEditController extends ManiphestController {
       // These allow task creation with defaults.
       if (!$request->isFormPost()) {
         $task->setTitle($request->getStr('title'));
+
+        $default_projects = $request->getStr('projects');
+        if ($default_projects) {
+          $task->setProjectPHIDs(explode(';', $default_projects));
+        }
       }
 
       $file_phids = $request->getArr('files', array());